Abdullah bin Umar reported,

The Prophet ﷺ reviewed him on the day of Uhud when he was fourteen years old and did not permit him to take part, then reviewed him on the day of the Trench when he was fifteen and permitted him.

Abu Dawud 2957

Sahih

حدثنا أحمد بن حنبل حدثنا يحيى عن عبيد الله أخبرني نافع

عن ابن عمر

أن النبي ﷺ عرضه يوم أحد وهو ابن أربع عشرة فلم يجزه وعرضه يوم الخندق وهو ابن خمس عشرة سنة فأجازه

أبو داود 2957صحيح
Contents
Reference
Commentary
Ayat
Collection
Sunan Abu Dawud
Book
Land tax, leadership, and spoils of war
Chapter
When a man is enrolled among the fighters
Numbering
Darussalam 2957
Abu Dawud 2957